Coating for the structured production of conductors on the surface of electrically insulating substrates

ABSTRACT

A coating for the structured production of conductors on the surface of electrically insulating substrates, in particular for producing sensor elements and printed circuit boards. The coating is formed from a doped tin oxide layer having the composition Sn 1- (y+z) A y  B z  O 2 , in which A is Sb or F and B is In or Al. The relative proportions in the coating of the dopants antimony (or fluorine) and indium (or aluminum) are defined by the limits 0.02&lt;y+z&lt;0.11 and satisfy the condition 1.4&lt;y/z&lt;2.2. The coating can be structured by ablation using electromagnetic laser radiation in the wavelength range 157-1064 nm. The creates an economical means for high resolution and waste-free structuring of insulating channels in thin, electrically conductive layers having high chemical, mechanical and thermal resistance on glass or ceramic substrates.

The invention relates to a coating for the structured production ofconductors on the surface of electrically insulating substrates,especially for the manufacture of sensor elements and printed circuitboards, wherein the coating is formed by a doped tin oxide layer withthe composition Sn₁₋(y+z) A_(y) B_(z) O₂ with A=Sb or F and B=In or Al,and to a method for applying the coating.

In chip and wafer production for microsensors, for hybrid circuits,displays, etc., the structured production of conductors plays a centralrole. In these areas so-called "masters," (masks) are used for thestructuring, which as a rule consist of glass material as the support.On this material a thin, homogeneous or inhomogeneous layer, of chromiumas a rule, is vapor deposited, which then must be structured with therequired layout. Also in many other areas of electronics, thestructuring of thin coatings or sandwiches on chiefly glass substratesis of interest with an view to their use as conductors and wiringsystems. It is already known to coat a thin metal layer about 0.1 μm to0.2 μm thick with a varnish, which is then exposed to an electron beamor laser beam or through optical systems and developed. Then a chemicaletching step follows which removes the exposed metal surfaces.

In the field of printed circuit board manufacture, DE 40 10 244 A1 hasdisclosed the application of a conductive varnish to a circuit board andthen working out the conductor pattern from the conductive varnish withaid of a laser. The thus produced conductor pattern is subsequentlymetallized.

DE 39 22 478 has already disclosed structuring plastic coated,copper-laminated base material with an excimer laser, i.e., removing theplastic coating in the areas where the conductive paths are thereafterapplied by electrolytic deposition of metal. Furthermore, in thissemi-additive process an alternative is envisioned of after depositionof the metal, removing the copper layer remaining between the conductorpaths with the excimer laser instead of by chemical etching. For thispurpose, however, an unrealistically high energy density is required,among other things. For this reason alone this process is unsuitable foreconomical fine structuring.

The use of full-surface tin oxide coatings for thin-layer heatingelements has been disclosed by EP 0 280 362 B1. A two-fold doping of tinoxide with antimony and indium is also described therein, which is saidto make it possible to use such coatings in thin-layer heating elementsat elevated temperatures. It is known that tin oxide coatings interactwith the atmosphere (O₂, H₂ O), especially at high temperatures, whichcan lead to considerable variations in their electric conductivity. Tobe able to use such coatings as electrically stable, transparentthin-layer heating elements even at high temperatures, antimony andindium are doped equimolarly, i.e., in the same amount. It is especiallypointed out that these amounts must not differ from one another by morethan 10%. The coatings furthermore have an antimony and indium contentof 4.5 mol-% each. The antimony thereby increases the conductivity, andthe indium stabilizes the crystal defects.

It is the object of the invention to provide a thin and electricallyconductive coating that can be applied to glass or ceramic substratesand in which insulating channels can be directly structured with highresolution and without residue.

Because the proportion of antimony or fluorine dopants to indium oraluminum in the coating is defined by the limits 0.02<y+z<0.11 and theratio of the dopants satisfies the condition 1.4<y/z<2.2, it issurprisingly achieved that such coatings are structurable withinsulating channels with high resolution and freedom from residue byablation by means of electromagnetic laser radiation in the wavelengthrange of 157 nm to 308 nm. It has furthermore been found that laserradiation in the infrared wavelength range up to 1064 nm can be used forthe high-resolution structuring of the coating according to theinvention. Such radiation can be produced, for example, by means ofdiode-pumped solid lasers. By means of an integrated frequency doubling,tripling or quadrupling, a shorter wavelength laser radiation can beproduced which likewise can be used advantageously for structuring thedescribed coating.

Furthermore, the object is achieved by a method which is characterizedin that the coating is applied in a thickness between 50 nm and 500 nmto an electrically insulating substrate of glass, ceramic or asemiconducting silicon at a surface temperature of 400° C. to 600° C. bymeans of an aerosol spray pyrolysis method at a temperature of 400° C.to 600° C.

The fine conductive structures produced by ablation by electromagneticlaser radiation in the specified wavelength range are characterized bysharp-edged vertical walls on the insulation channels, and the coatinghas an extraordinarily good resistance to chemical media as well asmechanical and thermal stresses. The coatings according to the inventioncan find application in sensors, e.g., for moisture sensors forautomobile windshields, and for conducting paths of microelectroniccircuits, especially for high-frequency components on a quartz glasssubstrate, and as displays, for example, in combination with LCDcoatings. The high conductivity and simultaneously high corrosionresistance are achieved by the special parameters of the coating processand structuring process, as well as especially by the addition ofantimony (Sb) and indium (In) to tin oxide in the stated concentrations.Alternatively, if the stated concentrations are maintained, fluorine canalso be used for antimony, and aluminum for indium.

It has proven to be especially advantageous if the coating has thecomposition: Sn₀.919 Sb₀.052 In₀.029 O₂. It has been found, a coating ofthis kind in particular has an optimum electrical conductivity and canbe removed by means of an excimer laser with high resolution, with noresidue and with extremely sharp edges. Applied to the surface of aglass substrate, such a coating is almost metallically conductive andtransparent, and also has an extraordinarily high corrosion resistanceand mechanical strength.

Preferably, the coating is applied at temperatures of 400° C. to 600° C.by means of an aerosol spray pyrolysis method. The thickness of thecoating lies between 50 nm and 500 nm. The subsequent structuring of thecoating is performed in a preferred embodiment of the invention byablation by means of a krypton fluoride excimer laser with a wavelengthof 248 nm.

In further embodiment of the invention, the substrate is specified to bequartz glass, glass ceramic, hard glass, especially borosilicate hardglass, soft glass, ceramic, especially aluminum oxide ceramic, orsemiconductive silicon.

In accordance with the invention it has become possible to use tin oxideas a semiconductor which has an energy band gap of 3.6 eV and is thustransparent to visible light, as a finely structurable coating, forsensors for example. Use is thereby made of the known effect that, byadding suitable dopants to tin oxide a density of freely movingelectrons (up to virtually 10²¹ per cm³) can be produced, which isunusually high for semiconductors. If thusly doped tin oxide isprecipitated in the form of thin layers on glass substrates, theseelectrically insulating glasses receive a coating which has a nearlymetallic conductivity and is simultaneously transparent.

It was found according to the invention that tin oxide coatings with acontent of preferably 5.2 mole-% antimony and 2.9 mole-% indium can bestructured precisely and without problems by means of excimer lasers,and even afterward they are stable electrochemically and mechanically.As it has been found, on the other hand, coatings doped equimolarly withantimony and indium present difficulties in regard to structuring withan excimer laser. Furthermore, the electrical conductivity and themechanical adhesion of the coatings now used according to the inventionare advantageously higher in comparison to coatings of the state of theart.

It is quite surprising that it is possible to ablate the coating usedaccording to the invention by means of an excimer laser in a finelystructured manner. Especially in the case of metals the economical useof excimer lasers for a clean ablation has appeared impossible due,among other things, to the high bonding energy. If the mechanisms ofinteraction of laser radiation and matter are considered, the laserwavelengths excite mainly vibration modes within the matter and thusheat metals, for example, to the molten or gaseous aggregate state.Lastly, there is a thermal ablation method which is not suitable forhigh-resolution structuring, especially also due to unavoidabledeposition of metals. The range of economical use of the excimer laserhas heretofore been limited exclusively to the ablation of polymers.Here considerable advantages can be achieved in ablation which resultfrom the high photon energies of this laser radiation. Unlike laserradiation that acts thermally, in the case of polymers the bondingenergy of the molecules is nullified and particles and monomers aresplit off cold. What is involved here is a non-thermal ablation process.

For the above reasons the advantages of the use of excimer lasers ondoped metal oxides were in no way to be expected. In particular, thesuccessful use of an excimer laser for the ablation of the tin oxidecoatings used according to the invention was not to be expected. At anyrate, the applicant's attempts toward the direct structuring of coatingssimilar in their properties, such as chromium nitride coatings, in orderto make a moisture sensor were unsuccessful. Instead it was found thatin this case, when an excimer laser is used, the evaporation of materialcan cause precipitation within the ablated areas. For this reason afunctioning sensor previously could not be produced by means of alaser-supported ablation of material.

As it has been found surprisingly, however, a pulsed excimer laser isvery well suited for the economical direct ablation of a newly developedspecial coating with the composition according to the invention, whichis disposed on a substrate of glass or ceramic. And structuring of thintin oxide coatings on glass or ceramic wafers is possible in the μmrange with extremely high resolution and without leaving any residue.The excimer laser thereby is used for the direct structuring of suchcoatings either with a mask disposed in the path of the beam havingareas transparent to the laser beam corresponding in shape andarrangement to the areas of the coating that are to be ablated, or elsea focused laser beam is used.

The invention will be further explained below with the aid of anexample.

The coating according to the invention was applied to the substrate bythe aerosol spray pyrolysis method. A spray solution was atomizedpneumatically to an aerosol by means of a commercial spray device. Thepropellant was dry nitrogen. The average diameter of the aerosoldroplets was within the range of a few μm. The spray nozzle was moved byan XY transport system at a distance of 10 cm to 15 cm perpendicular tothe glass substrate lying horizontally on a flat stove. The applicationwas made after the glass disks were cleaned with a known glass cleaner.The substrates were heated in an oven to a surface temperature of about550° C. Such a temperature proved to be especially advantageous, sincethe specific electrical conductivity of the applied coating generallyincreased markedly with rising temperature.

A spray solution according to the invention was prepared as follows:first 20 cm³ of tin chloride SnCl₄ was dissolved in 100 cm³ of n-butylacetate. Then 2.10 g of antimony chloride SbCl₃ and 1.14 g of indiumchloride InCl₃ were dissolved as dopants.

When applied the hot substrate surface, the chlorides contained in thesolvent are converted primarily by hydrolysis to an oxide film with theinventive composition Sn₀.919 Sb₀.052 In₀.029 O₂. Tin oxide filmthicknesses of 140 nm to 400 nm were thereby obtained.

A borosilicate glass plate was provided with a doped tin oxide coatingin a thickness of 140 nm. The surface resistance of this coating was 93Ω. This coating was further characterized by:

Crystal structure: polycrystalline, cassiterite (rutile structure)

Mean visible transmission: >85%

Refractive index: 1.95±0.03

Visual optical quality: clear, scatter-free, free of striae and pointdefects

Microscopically: free of cracks under 200 enlargement

Adhesion: resistant to adhesive tape and rubber eraser

Chemical stability: insoluble in water, dilute acids or alkalies(especially HNO₃, H₂ SO₄, HCl; NaOH, KOH), not attacked by aqueous saltsolutions (NaCl, KCl), resistant to common commercial glass cleaners,attacked only with difficulty in aqueous NaOH/KOH, resistant in the saltspray test per DIN

Surface resistance stability: stable in air up to at least 400° C., nodetectable change when wetted with water

Temperature coefficient of surface resistance: 3·10⁻⁴ K⁻¹ (referred to20° C.)

Microhardness: No measurable increase relative to hard glass (definitehardening detected on soft glass)

The described tin oxide coating on a borosilicate substrate wasstructured by means of an excimer laser system in order to produce awetness sensor--in this case a passenger car windshield wiper washsensor.

The laser-supported structuring process is preferably characterized bythe use of an excimer laser in conjunction with a mask projection. Thelaser system used had the following technical specifications:

    ______________________________________    Laser type         KrF - Excimer laser    Wavelength         248 nm    Pulse length       3 to 25 ns    Max. pulse energy  460 mJ    Average power      70 W    Pulse peak power   >18 MW    Repeat rate        200 Hz    Divergence         2 × 3 mrad    Beam cross section (7-10) × 20 mm.sup.2    Energy stability   +/-4%    ______________________________________

Alternatively, it is possible to use an excimer laser in conjunctionwith a focusing system. Such an apparatus technique has the advantage ofvery flexible structuring. Basically, both the mask projection techniqueand also focussed processing with an excimer laser are possible. Thesetechniques are known in themselves. This also applies to the possibleuse of a mirror deflecting system.

The material ablation of the tin oxide coatings according to theinvention by means of an excimer laser proved to be problem free.Surprisingly, in particular, no precipitation of the vaporized coatingmaterial was observed in the insulating channel so that extremely finestructure could be produced, and no post-treatment was necessary. Withinthe insulation channels the substrate material was also ablated to adepth of 20 nm. The insulation channels within the tin oxide materialare characterized by virtually perpendicular walls and sharp exposededges. The surface resistance of the coating of the invention was about50 Ω to 150 Ω.

A manufactured automobile windshield washer sensor was sufficientlysensitive to detect even a partial covering with fog droplets. The testsfurthermore showed good resistance against dry abrasion, sufficientstability against commercial glass cleaners and car wash preparations,as well as sufficient durability in the salt-spray, environmentalcontamination and galvanic tests prescribed by the automobile industry.

In addition it was found that the specific electrical resistance of atin oxide coating applied in a thickness of 140 nm to borosilicate glasshad also slightly increased. This is to be attributed in part to thegrain sizes of the polycrystalline coating structure which increase withincreased coating thickness. Thus, in the case of a coating with athickness of 360 nm, a surface resistance of 30 Ω was measured insteadof the expected 36 Ω.

In view of the fact that laminated glass in automobile windshields isproduced composed of soda-lime glass (soft glass), it was also an objectof the invention to realize the above-described sensor on a soft glasssubstrate. The successful production of the sensor on soft glasssubstrates is described hereinafter as an additional embodiment of theinvention. Since only the coating process requires adaptation, only thesubstrate-specific differences in the coating process are described.

Soft glass has, among other things, a substantially greater content of aalkali oxide, especially Na₂ O in comparison with hard glass. Beginningat temperatures above 450° C. sodium, as a small ion, is known todiffuse to a considerable degree at the surface of the soft glass. If ata surface temperature according to the inveniton of 500° C. to 550° C.,a coating with tin oxide is commenced, an sodium-containing intermediatecoating forms which, due to sodium's property of binding free electronsin tin oxide, has a high specific electrical resistance. At low coatingrates a sodium-containing tin oxide coating thus is formed, which atcoating thicknesses in the range of 100 nm can lead to in surfaceresistances in the megohm range. The existence of such transitioncoatings has been established spectrophotometrically.

If, however, a high coating rate of about 52 nm/min is chosen, at acoating time of 7 minutes, for example, a low-resistance, 360 nm thickcoating with a surface resistance of (128±6) Ω is obtained. The coatingtook place in this case at a temperature of 500° C. By means of thiscoating a very effectively operating sensor for automobile windshieldswas produced. To prevent thermomechanical stresses from leading to afracture in the glass which is to be coated, it appears advantageous inthe glass industry to apply the coating immediately during the coolingphase of the production of float plate glass. The problematic reheatingof soft glass sheets can thus be avoided.

Basically, the field of use of the invention is in no way limited tomoisture sensors. It is also possible to produce conductive paths forelectronic circuits. If desired, it is then also possible additionallyto apply a galvanic or even reductive coating of the tin oxide coatingaccording to the invention with conductive substances such as copper,gold, platinum, or the like. For conductive paths of electroniccircuits, a reinforcement of this kind may be advantageous in order toreduce the resistance of the path. The n-conductive tin oxide coatingsaccording to the invention form with these metals ohmic contacts of lowresistance. Moreover, the use of quartz glass with its low dielectriclosses is advantageous as a substrate for the construction of conductivestructures for high-frequency circuits. In general, the tin oxidecoatings according to the invention form a very stable bond with all ofthe substrates herein named, since a strong chemical bond to thesurfaces in question is achieved.

An additional advantageous embodiment of the invention consists in theproduction of structures on soft glass and hard glasses for displayapplications. Structured tin oxide hereby serves as a transparentelectrode structure for liquid crystal films (LCDs). Furthermore, use inother electrostatically controlled displays is also possible. Lastly, italso is possible to use the described structured tin oxide coating as astructured heating element. Other sensor applications in addition tomoisture sensors include, for example, microstructured gas sensors. Inthese gas sensors in order to achieve selectivity for certain gases, theindividual cells must be heated differently.

We claim:
 1. An article of manufacture comprising an electricallyinsulating substrate having deposited thereon a doped tin oxide layerhaving the composition:

    Sn.sub.1-(y+z) A.sub.y B.sub.z O.sub.2

wherein A represents at least one first dopant element selected from thegroup consisting of Sb and F, and B represents at least one seconddopant element selected from the group consisting of In and Al,andwherein the proportions of the dopants satisfy the relationships: 0.02<y+z<0.11and

    1.4<y/z<2.2;

said doped tin oxide layer being selectively ablatable by targetedelectromagnetic laser radiation having a wavelength in the range from157 nm to 1064 nm to produce structured conductive paths on thesubstrate.
 2. An article according to claim 1, wherein said doped tinoxide layer has the composition:

    Sn.sub.0.919 Sb.sub.0.052 In.sub.0.029 O.sub.2.


3. An article according to claim 1, wherein said doped tin oxide layerhas a thickness in the range from 50 nm to 500 nm.
 4. An articleaccording to claim 1, wherein said doped tin oxide layer is ablatable bytargeted excimer laser radiation having a wavelength in the range from157 nm to 308 nm.
 5. An article according claim 1, wherein said dopedtin oxide layer is ablatable by targeted krypton-fluoride excimer laserradiation with a wavelength of 248 nm.
 6. An article according to claim1, wherein the substrate is selected from the group consisting of quartzglass, glass ceramic, hard glass, soft glass, aluminum oxide ceramic andsemiconductive silicon.
 7. An article according to claim 6, wherein thesubstrate is a hard borosilicate glass.
 8. A method for producing acoated article comprising applying a layer of doped tin oxide by aerosolspray pyrolysis to an electrically insulating substrate at a surfacetemperature of 400° C. to 600° C., said layer having a thickness between50 nm and 500 nm, and said doped tin oxide having a composition:

    Sn.sub.1-(y+z) A.sub.y B.sub.z O.sub.2

wherein A represents at least one first dopant element selected from thegroup consisting of Sb and F, and B represents at least one seconddopant element selected from the group consisting of In and Al,andwherein the proportions of the dopants satisfy the relationships:

    0.02<y+z<0.11

and

    1.4<y/z<2.2.


9. A method according to claim 8, wherein said doped tin oxide layer hasthe composition:

    Sn.sub.0.919 Sb.sub.0.052 In.sub.0.029 O.sub.2.


10. A method according to claim 8, wherein the substrate is selectedfrom the group consisting of quartz glass, glass ceramic, hard glass,soft glass, aluminum oxide ceramic and semiconductive silicon.
 11. Amethod according to claim 8, wherein the substrate is manufactured by aprocess which includes a cooling phase, and said doped tin oxide layeris deposited on the substrate during said cooling phase.
 12. A method ofproducing a structured layer on a substrate, said methodcomprisingproviding an electrically insulating substrate having a layerof doped tin oxide deposited thereon, and ablating selected areas of thedoped tin oxide layer by exposing the selected areas to electromagneticlaser radiation having a wavelength in the range from 157 nm to 1064 nm;said doped tin oxide layer having the composition:

    Sn.sub.1-(y+z) A.sub.y B.sub.z O.sub.2

wherein A represents at least one first dopant element selected from thegroup consisting of Sb and F, and B represents at least one seconddopant element selected from the group consisting of In and Al,andwherein the proportions of the dopants satisfy the relationships:

    0.02<y+z<0.11

and

    1.4<y/z<2.2.


13. A method according to claim 12, wherein the doped tin oxide layerhas the composition:

    Sn.sub.0.919 Sb.sub.0.052 In.sub.0.029 O.sub.2.


14. A method according to claim 12, wherein the substrate is selectedfrom the group consisting of quartz glass, glass ceramic, hard glass,soft glass, aluminum oxide ceramic and semiconductive silicon.
 15. Amethod according to claim 12, wherein the doped tin oxide layer has athickness in the range from 50 nm to 500 nm.
 16. A method according toclaim 12, wherein the selected areas of the doped tin oxide layer areexposed to excimer laser radiation having a wavelength in the range from157 nm to 308 nm.
 17. A method according claim 12, wherein the selectedareas of the doped tin oxide layer are exposed to krypton-fluorideexcimer laser radiation having a wavelength of 248 nm.
 18. An article ofmanufacture comprising an electrically insulating substrate having astructured layer of doped tin oxide thereon produced by the process ofclaim
 12. 19. An article of manufacture according to claim 18, whereinsaid article comprises a moisture sensor.
 20. An article of manufactureaccording to claim 19, wherein said substrate comprises an automobilewindshield.